Essentially, animals are many-celled heterotrophic organisms. Plants and algae characteristically manufacture their food from inorganic substances (usually by photosynthesis); animals must secure food already organized into organic substances. They are dependent upon photosynthetic organisms, which provide oxygen as a byproduct and are the ultimate source of all their food. Animals (as well as plants) provide carbon dioxide through respiration and the decomposition of their dead bodies (see carbon cycle; nitrogen cycle). In addition, most animals have specialized means of locomotion, generally involving muscle cells, and possess nervous systems and sense organs—all adaptations for securing food. In most forms there is a distinct alimentary canal or digestive system. Animal cells do not have cell walls. Almost all animals, unlike most plants, possess a limited scheme of growth; that is, the adults of a given species are nearly identical in their characteristic form and are similar in maximum size. Most animals reproduce sexually, but some are capable of asexual reproduction under certain circumstances.
With the advent of electron microscopy and advanced biochemical analyses, intricate differences between simple and microscopic organisms were better understood, and many that were previously fit into the animal or plant kingdom were then placed into separate kingdoms (i.e., Monera for the bacteria, Protista for the algae and protozoans, and so forth). In zoological classification the animal kingdom has been divided into the three subkingdoms of Parazoa (the sponges), Mezozoa (wormlike parasites), and Eumetazoa. Eumetazoa comprises numerous invertebrate phyla and the phylum Chordata. The chordates include two primitive subphyla of a few species each and the subphylum Vertebrata (see vertebrate). There are at least 1.5 million animal species; approximately 95% of these are invertebrates.
The scientific study of animals is called zoology; the study of their relation to their environment and of their distribution is animal ecology. For specific approaches to the study of living things, see biology.